Bistable Systems and Molecular Switching

Supervisor: Prof.Davor Boghai By: Seyyed Mohammad Reza Sanavi Hosseini

Bistable Systems Any molecular-level system that can be reversibly switched between two different states by use of an external stimulus can be taken as a basis for storing information, i.e. for memory purposes. An ideal molecular-level memory should be stable and easy to write, and its switched form should be stable, easy to read, and erasable when necessary. Systems that undergo an irreversible change can be used as permanent memories (e.g. photography, dosimetry). In more complex systems switching can be performed among more than two states. This possibility can be exploited to obtain memories which are permanent unless they are erased on purpose, or for performing logic operations. Any molecular-level system that can be reversibly switched between two different states by use of an external stimulus can be taken as a basis for storing information, i.e. for memory purposes. An ideal molecular-level memory should be stable and easy to write, and its switched form should be stable, easy to read, and erasable when necessary. Systems that undergo an irreversible change can be used as permanent memories (e.g. photography, dosimetry). In more complex systems switching can be performed among more than two states. This possibility can be exploited to obtain memories which are permanent unless they are erased on purpose, or for performing logic operations.

Important types of stimulus 1) Light energy (photons) 2) Electrical energy (electrons/holes) 3)Chemical energy ( in the form of protons, metal ions, etc.)

Note: In photochemical stimulation the most common switching processes are related to photoisomerization or photoinduced redox reactions; for electrochemical inputs, the induced processes are, of course, redox reactions.

Comparison between different types of stimulation Compared whit chemical stimulation, photochemical and, to some extent, electrochemical stimuli can be switched on and off easily and rapidly. A further advantage of the use of photo chemical and electro chemical techniques is that photons and electrons, besides supplying the stimulus to make a switch work (i.e. ” to write ” the information bit), can also be useful “ to read ” the state of system and thus to control and monitor its operation. It should be also noted that photochemical and electrochemical inputs and outputs are among the easiest to interface to microscopic systems, making them amenable to the multi-scale engineering required for the eventual creation of real devices

Significant switching systems 1)Photochromic systems: The term “ photochromic ” is applied to molecules that can be reversibly inter converted, with at least one of the reactions being induced by light excitation, between two forms whit different absorption spectra. The two forms, differ not only in their absorption spectra, but also in several other properties such as redox potentials, dielectric constant, etc.

Representation of a photochromic system (a) and of its energy profile (b)

Some important families of photochromic compounds: a) Diarylethenes b) Flugides c) Spiropyrans d) Azobenzenes e) dihydroazulenes

2) Modulation of Host-Guest interactions: Switching host-guest interactions by means of photochromic reactions might lead to a variety of sensors and to transport of guest molecules across a membrane. An example is shown next page.

Photoswitchable recognition of saccharides by diarylethene derivative 1

3)Fluorescent Switches: Several photochromic compounds have different fluorescence properties in their two forms. Occasionally on/off switching is observed. Two examples is shown next page

On/off switching of fluorescence in (a) fulgide derivative 3 and (b) diaryiethene derivative

4) Chiroptical Switches: Chiral photobistable molecules are a particularly interesting class of photochromic compounds. In such molecules reversible photochemical trans formation can lead to a change in chirality. The importance of chiroptical switches is further emphasized since chirality controls most natural chemical processes, including molecular recognition, transport, information storage, catalysis, assembly, and replication. Chiral switches based on photochromic molecules can be subdivided as follows:

a) Switching of enantiomers b) Switching of diastereoisomers c) Functional chiral switches d) Switching of macromolecules or supramolecular organization Besides the general requirements needed for photochromic switches, chiroptical switches must be stable towards thermal racemization.

Thermal and photochemical isomerization processes of chiroptical switch

5)Photochemical Biomolecular Switches: It is well known that many biological processes are triggered by light signals. Such systems consist of a biological material or an environment the innate function of which can be activated/deactivated by artificial photoresponsive units.

6) Electrochromic Systems: Electrochromic is applied to compounds that can be inter converted, by reversible redox processes, between two forms whit different absorption spectra. In electrochromic systems the interconverting species are not isomers, because they have different number of electrons. In these systems several successive switching processes can occur.

7) Redox Switches: an example of redox switch is shown below

Multistate-Multifunctional Systems 1)Biphotochromic Supermolecular Systems: Many attempts have been made to couple two photochromic units in the same supramolecular species. The objective of these studies was to obtain a synergistic effect between the properties of the two units to create materials with novel photokinetic properties.

2)Photochemical Inputs Coupled whit Other Stimuli: a) Three-state Systems. Write-Lock-Read-Unlock-Erase Cycle b) Orthogonal photochemical-Electrochemical Stimulation c) Orthogonal Photochemical-(Acid-Base) Stimulation d) Molecular Shift Register

Schematic representation of the behavior of three types of photochromic systems

3) Multielectron Redox Processes: a) System whit Equivalent redox Units b) System whit Nonequivalent Redox Units 4) Electrochemical inputs Coupled whit Chemical Inputs 4) Electrochemical inputs Coupled whit Chemical Inputs 5) Multiple Chemical Inputs 5) Multiple Chemical Inputs

Photochemical and electrochemical switching of a diarylethene derivative to perform a write- lock-read-unlock-erase cycle

Schematic representation of the write-lock- read-unlock-erase cycle of a system

Light-and redox- driven switching of the DHA-VHF system

Photo-switching materials in following parts we can see some examples and applications of molecular switching.

Photo-magnetic molecular solids 1)Prussian blue analogues and CN-bridging compounds a) a) Fe – Co Prussian blue b) b) CN-bridging compounds

(Right) Structure and (left) field-cooled magnetization curves at 5G

(Top) View of the molecular structure, a projection of the three- dimensional crystal packing, and (bottom) magnetic susceptibilities (MT) vs. temperature before and after UV light illumination for Nd(DMF)4(H2O)3(- CN)Fe(CN)5 · H2O

2)Light-induced excited spin state trapping compounds (Top) View of molecular structure and (bottom) temperature dependence of the magnetization before and after light illumination for [FeIII(pap)2] · ClO4

Photo-chromic magnetic materials Photo-controllable magnetic vesicles: the first successful example of the preparation of photo-chromic magnetic (composite) materials. A commercially available double-chain ammonium amphiphile, didodecyldimethylammonium bromide ([CH3(CH2)11]2(CH3)2N+Br-), which forms a vesicle structure in aqueous solution by sonication, and an azobenzenecontaining amphiphile (C12AzoC5N+Br-), which was synthesized according to a reported procedure, were mixed with an aqueous solution of poly(vinyl alcohol) (PVA). The photo-chromic vesicle films were prepared by casting the above solution on a clean glass plate at room temperature

Design of photo- responsive magnetic vesicles containing Prussian blue and azobenzene

Photo-switchable magnetic films 1)Prussian blue intercalated in Langmuir – Blodgett films consisting of an amphiphilic azobenzene and a clay Mineral. 2) Anisotropic photo-induced magnetization effect in ultrathin Fe – Co Prussian blue films

Preparation of the hybrid multilayered films

Schematic representation of photo-switchable magnetic layer-by-layer films. (Bottom) Changes in the magnetizatio n for the films induced by alternating illumination with UV and visible light at 300K at 10G.

Molecular models of the gemini peptide lipid 1 evaluated by molecular mechanism calculation using the Cerius2 software based on DREIDING force field: (a) trans-form; (b) cis- form bound a metal ion.

An example of data storage Memory operation of the single-molecule system. The blue line shows the write, read, and erase pulse pattern applied and the red line demonstrates the resulting switching between "off" and "on" states of the molecular system. As can be seen in the picture, the system is initially in the "off" state. Then a write pulse of +1.6 V is applied and the molecule switches to the "on" state. This state can be read out using a voltage of +1.1 V. The molecule can be switched back to the "off" state by another pulse (erase pulse) of -1.6 V. ( Source: IBM)

References: 1)Journal of Photochemistry and Photobiology C: Photochemistry Reviews7 (2006) 69 – 88 2)Electrochemistry Communications 9 (2007) 173 – 179 3)Journal of Photochemistry and Photobiology A: Chemistry 183 (2006) 309 – 314 4)Journal of Luminescence 119 – 120 (2006) 478 – 481 5)Composites: Part A 38 (2007) 747 – 754 6) 7)Molecular Devices and Machines, a Journey into the Nanoworld,V.Balzani, M.Venyuri, A.Credi

The End Thanks for your attention